Hi David > -----Original Message----- > From: David Hildenbrand <[email protected]> > Sent: Wednesday, July 29, 2020 2:37 PM > To: Justin He <[email protected]> > Cc: Dan Williams <[email protected]>; Vishal Verma > <[email protected]>; Mike Rapoport <[email protected]>; David > Hildenbrand <[email protected]>; Catalin Marinas <[email protected]>; > Will Deacon <[email protected]>; Greg Kroah-Hartman > <[email protected]>; Rafael J. Wysocki <[email protected]>; Dave > Jiang <[email protected]>; Andrew Morton <[email protected]>; > Steve Capper <[email protected]>; Mark Rutland <[email protected]>; > Logan Gunthorpe <[email protected]>; Anshuman Khandual > <[email protected]>; Hsin-Yi Wang <[email protected]>; Jason > Gunthorpe <[email protected]>; Dave Hansen <[email protected]>; Kees > Cook <[email protected]>; [email protected]; linux- > [email protected]; [email protected]; [email protected]; Wei > Yang <[email protected]>; Pankaj Gupta > <[email protected]>; Ira Weiny <[email protected]>; Kaly Xin > <[email protected]> > Subject: Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem > alignment > > > > > Am 29.07.2020 um 05:35 schrieb Jia He <[email protected]>: > > > > When enabling dax pmem as RAM device on arm64, I noticed that kmem_start > > addr in dev_dax_kmem_probe() should be aligned w/ > SECTION_SIZE_BITS(30),i.e. > > 1G memblock size. Even Dan Williams' sub-section patch series [1] had > been > > upstream merged, it was not helpful due to hard limitation of kmem_start: > > $ndctl create-namespace -e namespace0.0 --mode=devdax --map=dev -s 2g -f > -a 2M > > $echo dax0.0 > /sys/bus/dax/drivers/device_dax/unbind > > $echo dax0.0 > /sys/bus/dax/drivers/kmem/new_id > > $cat /proc/iomem > > ... > > 23c000000-23fffffff : System RAM > > 23dd40000-23fecffff : reserved > > 23fed0000-23fffffff : reserved > > 240000000-33fdfffff : Persistent Memory > > 240000000-2403fffff : namespace0.0 > > 280000000-2bfffffff : dax0.0 <- aligned with 1G boundary > > 280000000-2bfffffff : System RAM > > Hence there is a big gap between 0x2403fffff and 0x280000000 due to the > 1G > > alignment. > > > > Without this series, if qemu creates a 4G bytes nvdimm device, we can > only > > use 2G bytes for dax pmem(kmem) in the worst case. > > e.g. > > 240000000-33fdfffff : Persistent Memory > > We can only use the memblock between [240000000, 2ffffffff] due to the > hard > > limitation. It wastes too much memory space. > > > > Decreasing the SECTION_SIZE_BITS on arm64 might be an alternative, but > there > > are too many concerns from other constraints, e.g. PAGE_SIZE, hugetlb, > > SPARSEMEM_VMEMMAP, page bits in struct page ... > > > > Beside decreasing the SECTION_SIZE_BITS, we can also relax the kmem > alignment > > with memory_block_size_bytes(). > > > > Tested on arm64 guest and x86 guest, qemu creates a 4G pmem device. dax > pmem > > can be used as ram with smaller gap. Also the kmem hotplug add/remove > are both > > tested on arm64/x86 guest. > > > > Hi, > > I am not convinced this use case is worth such hacks (that’s what it is) > for now. On real machines pmem is big - your example (losing 50% is > extreme). > > I would much rather want to see the section size on arm64 reduced. I > remember there were patches and that at least with a base page size of 4k > it can be reduced drastically (64k base pages are more problematic due to > the ridiculous THP size of 512M). But could be a section size of 512 is > possible on all configs right now.
Yes, I once investigated how to reduce section size on arm64 thoughtfully: There are many constraints for reducing SECTION_SIZE_BITS 1. Given page->flags bits is limited, SECTION_SIZE_BITS can't be reduced too much. 2. Once CONFIG_SPARSEMEM_VMEMMAP is enabled, section id will not be counted into page->flags. 3. MAX_ORDER depends on SECTION_SIZE_BITS - 3.1 mmzone.h #if (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS #error Allocator MAX_ORDER exceeds SECTION_SIZE #endif - 3.2 hugepage_init() MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER >= MAX_ORDER); Hence when ARM64_4K_PAGES && CONFIG_SPARSEMEM_VMEMMAP are enabled, SECTION_SIZE_BITS can be reduced to 27. But when ARM64_64K_PAGES, given 3.2, MAX_ORDER > 29-16 = 13. Given 3.1 SECTION_SIZE_BITS >= MAX_ORDER+15 > 28. So SECTION_SIZE_BITS can not be reduced to 27. In one word, if we considered to reduce SECTION_SIZE_BITS on arm64, the Kconfig might be very complicated,e.g. we still need to consider the case for ARM64_16K_PAGES. > > In the long term we might want to rework the memory block device model > (eventually supporting old/new as discussed with Michal some time ago > using a kernel parameter), dropping the fixed sizes Has this been posted to Linux mm maillist? Sorry, searched and didn't find it. -- Cheers, Justin (Jia He) > - allowing sizes / addresses aligned with subsection size > - drastically reducing the number of devices for boot memory to only a > hand full (e.g., one per resource / DIMM we can actually unplug again. > > Long story short, I don’t like this hack. > > > > This patch series (mainly patch6/6) is based on the fixing patch, ~v5.8- > rc5 [2]. > > > > [1] https://lkml.org/lkml/2019/6/19/67 > > [2] https://lkml.org/lkml/2020/7/8/1546 > > Jia He (6): > > mm/memory_hotplug: remove redundant memory block size alignment check > > resource: export find_next_iomem_res() helper > > mm/memory_hotplug: allow pmem kmem not to align with memory_block_size > > mm/page_alloc: adjust the start,end in dax pmem kmem case > > device-dax: relax the memblock size alignment for kmem_start > > arm64: fall back to vmemmap_populate_basepages if not aligned with > > PMD_SIZE > > > > arch/arm64/mm/mmu.c | 4 ++++ > > drivers/base/memory.c | 24 ++++++++++++++++-------- > > drivers/dax/kmem.c | 22 +++++++++++++--------- > > include/linux/ioport.h | 3 +++ > > kernel/resource.c | 3 ++- > > mm/memory_hotplug.c | 39 ++++++++++++++++++++++++++++++++++++++- > > mm/page_alloc.c | 14 ++++++++++++++ > > 7 files changed, 90 insertions(+), 19 deletions(-) > > > > -- > > 2.17.1 > >
